1. Field of the Invention
The present invention relates to a method of joining by use of an Au—Sn brazing material, for example, in joining a lead pin to a circuit terminal or sealing a semiconductor package.
2. Description of the Related Art
Au—Sn brazing materials are brazing materials in which eutectic structures of Au—Sn alloys are used, and have relatively low melting points (about 280° C.). In addition, Au—Sn brazing materials are excellent in corrosion resistance and have the advantage that they do not contain harmful substances such as Pb unlike solders (Sn-37 wt % Pb) that have thitherto been used. Au—Sn brazing materials are widely used in the field of electrical and electronic equipment and have been field proven, for example, as sealing materials for the hermetic sealing of electronic part packages and joining materials for joining lead pins to circuit terminals.
Although basically a joint formed by an Au—Sn brazing material has an Au—Sn eutectic structure, an Au—Sn alloy phase having a high Au content (hereinafter referred to as an Au rich phase) may sometimes occur partially. This Au rich phase, which is a kind of intermetallic compound, is hard and has a higher melting point than the surrounding Au—Sn eutectic phase. Therefore, this Au rich phase does not dissolve at the melting points of brazing materials and remains as a solid phase. And because the Au rich phase is diverse in size, variations may sometimes be produced in the thickness of a joint. In particular, in a joining method using Au—Sn brazing materials, joining is often performed by plating parts to be joined with gold in order to improve wettability. Owing to the diffusion of gold from this gold plating, the gold content in a joint rises and an Au rich phase becomes apt to occur. Also, in the composition range in which this Au rich phase occurs, the crystal structure becomes coarsened and the solder flow becomes worse, thus posing a problem.
The present applicant has so far disclosed a sealing method and a joining method that are effective in the above-described uses (Japanese Patent No. 3086086, Japanese Patent Laid-Open No. 2001-176999). In these methods, compared to Au—Sn brazing materials of Au-20 wt % Sn that have hitherto been used, there are used brazing materials having a composition slightly displaced from a eutectic point, in which the gold content is lowered and the Sn content is raised to 20.5 through 21.5%. In these methods, by using a brazing material of low gold content, the composition of the brazing material is prevented from deviating from a eutectic point to the gold side even when the gold content in the brazing material rises during joining due to the diffusion of gold from outside and the occurrence of a gold rich phase is suppressed. A joint formed by these methods has a complete eutectic structure or a eutectic structure containing a small amount of Sn rich phase. Compared to an Au rich phase, an Sn rich phase has a low melting point, exerts a small effect and suppresses the coarsening of a structure by suppressing the occurrence of an Au rich phase. Therefore, by causing a joint having such a structure like an Sn rich phase to be formed, it becomes possible to perform joining in a stable manner.
The methods disclosed by the present applicant can efficiently form good joints although the methods are simple ones that involve slightly changing the composition of a brazing material to be used. However, according to examinations by the present inventor, although in most cases good joints are obtained by using this brazing material the composition of which is adjusted, in some cases an Au rich phase occurs and satisfactory joints may not be formed. Although the fraction defective itself on that occasion is not high, from the standpoint of ensuring product yield and effective utilization of resources, it is desirable to cause the fraction defective to approach zero as far as possible if this is possible.
The present invention has been made against the above-described background and has as its object the provision of a joining method using an Au—Sn brazing material that completely suppresses the occurrence of an Au rich phase and can form a homogeneous joint with higher probability.